The invention is concerned with a form of photosensitively colorable glass known as polychromatic glass. This is a transparent glass capable of having developed therein a range of colors that essentially span the color spectrum. Color is developed in a polychromatic glass by certain combinations of radiation exposure and thermal treatment. Variation of color within a given glass results primarily from variation in the initial exposure flux which is defined as the product of radiation intensity and exposure time.
U.S. Pat. No. 4,017,318, granted Apr. 12, 1977 to Pierson and Stookey, describes the physical and chemical characteristics of polychromatic glasses; also, general methods of developing a range of colors in such glasses. The disclosure of that patent, in its entirety, is incorporated herein by reference.
As disclosed in the Pierson-Stookey patent, polychromatic glasses can be composed of a wide range of base compositions. However, each must contain silver, an alkali metal oxide (preferably Na.sub.2 O), fluoride, and at least one halide selected from the group of chloride, bromide, and iodide. The glasses may be irradiated with either high energy or actinic radiations. Where the actinic radiation is supplied as ultraviolet radiation, cerium oxide (CeO.sub.2) is a required component of the glass composition.
In one of the methods described in the patent, the glass is initially exposed to high energy or actinic radiations selected from the group of high velocity electrons, X-radiations, and actinic radiations in the ultraviolet portion of the spectrum, preferably in the range of about 2800 A-3500 A. This exposure develops a latent image in the glass. The intensity and time of the exposure primarily determine the final color which will be produced in the glass.
Thereafter, the glass is subjected to a heat treatment at a temperature between its transformation range and its softening point. This causes precipitation of colloidal silver particles in situ to act as nuclei. Where a transparent final product is desired, the heat treatment is of such duration as to effect the precipitation of colloidal silver nuclei and to cause the growth thereon of extremely small microcrystals of alkali metal fluoride-silver halide, e.g., NaF+AgCl and/or AgBr and/or AgI. If an opal glass product is sought, the heat treatment will be continued for a sufficient length of time to not only promote the precipitation of colloidal silver nuclei, but also to effect the growth of said microcrystals on the silver nuclei to a size large enough to scatter light.
The nucleated glass is then cooled conveniently to room temperature but, in any event, to a temperature at least 25.degree. C. below the strain point of the glass. Then, it is again exposed to high energy or actinic radiations. This second exposure intensifies the color, the hue of which was previously determined via the first exposure. Finally, the glass is reheated to a temperature between about the transformation range and the softening point of the glass to produce the desired color in the glass. It has been theorized that submicroscopic particles of metallic silver are precipitated as discrete colloidal particles and/or deposited on the surface and/or within the alkali metal fluoride-silver halide microcrystals.
The mechanism of the color phenomenon is not undisputably known. However, the quantity of silver precipitated and the geometry thereof, as well as, perhaps, the refractive index of the crystals, are deemed to determine the color produced. In any event, the colors are achieved with very low silver contents and exhibit characteristics similar to interference colors. Hence, it was surmised that at least one of the three following circumstances is present: (1) discrete colloidal particles of silver less than about 200 A in the smallest dimensions; (2) metallic silver deposited within alkali fluoride-silver halide microcrystals, the silver-containing portion of the microcrystals being less than about 200 A in the smallest dimensions; and (3) metallic silver deposited upon the surface of said microcrystals, the silver-coated portion of the microcrystals being less than about 200 A in the smallest dimension.
The patent discloses that the heat treatment after each exposure to high energy or actinic radiation may consist of a series of heatings and coolings rather than a single heat treating cycle. This does not change the color developed, but can improve color intensity.
The patent further teaches a sequence of colors that may be developed by progressively increasing the magnitude of the exposure flux in the initial exposure step. Thus, progressively increasing the radiation flux (by increasing intensity, time, or both) successively produces pale yellow, green, blue, violet, red, orange, and dark yellow colors.
U.S. Pat. No. 4,092,139, granted May 30, 1978 to J. Ference, discloses an alternate color developing procedure whereby the second exposure (the re-exposure) of the Pierson-Stookey method is carried out at a temperature between 200.degree. and 410.degree. C. to cause the metallic silver to precipitate. This obviates the separate reheating step. Also, U.S. Pat. No. 4,134,747, granted Jan. 16, 1979 to Pierson and Stookey, describes a procedure wherein the potentially polychromatic glass is melted in a reducing atmosphere. During the color development procedure, this modification produces a so-called reverse opal effect. Thus, exposed portions of the glass remain transparent, although colored, and unexposed portions of the glass become opacified.
The earlier Pierson-Stookey patent (318) indicates that the exact reaction path leading to coloration is not known with certainty. However, a proposed explanation is that silver nuclei are photosensitively precipitated, using cerous ions as a photosensitizing agent, as illustrated by this equation: Ag.sup.+ +Ce.sup.+3 +hv.fwdarw.Ag.degree.+Ce.sup.+4. The nucleation and growth of complex silver plus alkali metal (Na) halide crystals is then proposed as follows: ##EQU1## C is a crystallite of the nature (Ag.degree.).sub.n x(Na.sup.+ +Ag.sup.+)X and X.sup.- is a halide ion. The development of the colored species, resulting from a second radiation step and second heat treatment, is then expressed as: ##EQU2## CmAg.degree. is a crystal having granular particles of metallic (m) silver formed on, or dispersed within, it.
The patent concludes that, when ultra-violet light is used as the source of radiation, at least about 0.01% CeO.sub.2 is required as a photosensitizing agent. To demonstrate this requirement, the patent sets forth comparative Examples 17 and 18. The glasses of these examples are identical in composition and treatment, except that the glass of Example 18 contains CeO.sub.2 and that of Example 17 does not. Similarly, the glass of Example 18 is reported to function as a polychromatic glass while that of Example 17 does not.